Lead-in bushings



Patented Jan. 13, 1959 LEAD-IN BUSHINGS Frank J. Corey, Belmont, Mass., assignor to Raytheoii Manufacturing Company, Waltham, Mass, a corporation of Delaware Application August 4, 1955, Serial No. 526,349

3 Claims. (Cl. 174-453) This invention relates to a lead-in bushing, and particularly to a bushing for providing a high electrical insulation and an hermetic seal to a terminal or other conducting member at its point of entry into a case, container, or similar. housing.

In many types of electrical apparatus, for example, transformers, it is desirable or necessary to enclose some element, such as a coil, in a housing. Frequently, the housing should be hermetically sealed to prevent the loss of fluid flowing, such as oil, from a container, or to prevent the entrance thereto of air or moisture from the atmosphere. The lead-in connections for such apparatus must be well insulated from the metallic walls through which they pass, while at the same time the opening must be sealed against leakage even at considerable differences of pressure between the interior and exterior of the container.

In this invention, there is disclosed a lead-in bushing comprising a member of resilient material, such as silicone rubber, having a passage extending longitudinally therethrough. An electric conductor having a diameter greater than the normal contracted diameter of said passage, but sufficiently small to permit the conductor to be forced through said passage without exceeding the elastic limits of said material, is placed through said passage of said material. An outer shell having an interior surface complementing the shape and fitting closely with an exterior surface of said resilient member when said member is in its normally unexpanded state is placed over said conducting member. Said outer shell has a camming surface in contact with a cammed surface on said resilient member and, in addition, said inner surface of said outer shell and said outer surface of said member form an arrangement for lengthening the leakage path along said inner surface, One such arrangement is made by having said outer shell forming a U- shaped protrusion and said outer surface of said member forming a suitable receptacle for accepting said U- shaped protrusion. Means cooperating with said conductor is used for compressing said outer shell against said member.

Further objects of this invention will be apparent as the description progresses, reference now being made to the accompanying drawing wherein:

Fig. l is a cross-sectional view of a first embodiment of this invention; and

Fig. 2 is a cross-sectional view of a second embodiment of this invention.

Referring now to Figs. 1 and 2, there is shown a leadin bushing comprising a resilient member 11, such as silicon rubber. The resilient member 11 is placed through a hole 12 in a panel 13. Placed through the passage in said resilient member 11 is a conducting member 14; said conducting member is constructed preferably of a diameter greater than the normal contracted diameter of the passage in said resilient member 11, but sufficiently small to permit said conductor to be brought through said passage without exceeding the elastic limits of said resilient member.

' member 11 is an outer shell 15 and 16 constructed of an insulating material, preferably ceramic. Outer shell 15 and 16 has an interior surface complementing the shape and fitting closely with the exterior surface of said resilient member 11 when said member is in its normally unexpended state. shells 15 and 16 form U-shaped protrusions 17 and 18, and said outer surface of said resilient member 11 forms suitable receptacles for accepting said U-shaped protrusions. In addition, outer shell 15 has a camming surface- 19 in contact with a cammed surface 20 on said resil-' ient member 11. Located on said conducting member 14 are a plurality of grooves constructed in the form of a thread 21 so placed as to be in contact with the resilient member 11. One end of conducting member 14 receives'nut 22, which serves the dual purpose of connecting lead Wire 23 to said conducting member 14 and to also supply the necessary compressing forces for maintaining the lead-in bushing as a complete assembly,-

At the other end ;of conducting member 14 opposite. nut 22 is a stud 24 to which external wires, not illustrated, may 'be connected, thereby completing a circuit from said external wires to said lead wire 23.

The addition of the male and female portions to the outer insulator shell and to the inner gland or resilient member in a mating and matching position on either top or bottom, or both, contributes the aforementioned new and advantageous features. Due to practices common in the.ceramic and in the rubber art, and in the nature of the materials used, neither can be produced in the required form without expensive grinding operations to the very close tolerances needed to achieve the fit nec essary between said mating parts which is necessary for the proper functioning of the terminal bushing. Shrinkage of either or both materials is a factor difficult to control and when assembled parts happen to be in opposite extremes of commercially feasible tolerance limits, a misfit occurs and the malfunction of the bushing usually follows. In a bushing constructed accordto the principles of this invention, the above-mentioned condition is alleviated in that a mismatch of diameters that may cause a loose fit in previously constructed bushings will now cause a tightness to the added secondary mating parts. The gland, being of resilient material, thus adjusts itself to a good fit not attainable in old arrangements under the same conditions of misfit when assembled. In addition, the male and female mating surfaces add to the length of the breakdown path within the insulator and thereby permit a given size of internal gland to withstand greater electrical stresses than heretofore possible. Because of this longer breakdown path, it is possible to use a gland of a fixed size in more than one size of terminal bushing as illustrated in Figs. 1 and 2 as resilient members 11.

This completes the description of the embodiment of the invention illustrated herein. However, many modifications and advantages thereof will be apparent to persons skilled in the art without departing from the spirit and scope of this invention. Accordingly, it is desired that this invention not be limited to the particular details of the embodiment disclosed herein, except as defined by the appended claims.

What is claimed is:

l. A lead-in bushing comprising a member of resilient insulating material having a passage extending longitudinally therethrough, an electric conductor having a diameter greater than the'normal contracted diameter of said passage forced through said passage without exceeding the elastic limits of said material, an outer shell of insulating material having an interior surface complementing the shape and fitting closely with an exterior Covering the resilient.

The inner surfaces of said outer.

surface of said resilient member when said member is i in its normally unexpanded state, a portion of said said inner surface of said outer shell forming a U-shaped protrusion and a portion of said outer surface of said member forming a suitable receptacle. for accepting said.

ceeding the elastic. limitsof said material, :said conductor" having grooves located .on that part of said conductor which-is in contact with. said member, an outer shell of insulating material .having an interior surface complementing the shapeand .fitting closely with an exterior surface of said resilient member when said member is in its normally unexpanded state, saidouter shell having a camming surface in contact with a .cammed surface on said-member, a portion of said inner surface of said outer shell forming a U-shaped protrusion and a portion of'said. outer-surface of said member forming a suitable receptacle for accepting said U-shaped protrusion, and means for compressing said outer shell against said member.

3. A lead-in'bushing comprising a member of resil- 'ient insulating material having a passage extending longitudinally therethrough, an electric conductor having a diameter greater than the normal contracted diameter of said passage forced through said passage without exceeding the elastic limits of said material, an outer shell comprising a pair of cup-shaped members of nonresilient nonconductive material having interior surfaces complementing the shape and fitting closely with an exterior surface of said resilient member when said member is in its normally unexpanded state, one of said cup-shaped members having a camming surface in contact with a cammed surface on said member, a portion of said inner surfaces of both of said outer shells forming a U-shaped protrusion and a portion of said outer surface of said member forming a suitable receptacle 'for accepting both of said U-shaped protrusions, and means for compressing both of said shells against said member.

References Cited in the file of this patent UNITED STATES PATENTS 1,878,229 Beck Sept. 20, 1932 2,402,927 Stupakoff June 25, 1946 2,447,489 Clark Aug. 24, 1948 2,476,074 I Unger July 12, 1949 2,513,080 Burtt June 27, 1950 2,704,355- Holton Mar. 15, 19,55 

